Here’s some info on colace use in humanshttps://www.everydayhealth.com/drugs/colace
Seems they say not to use it long term because the body will become dependant on it. If it’s the only thing that works for him though, you may need to. Have you asked your vet about cisapride? I haven’t used it, but it was the next step for my civie if the lactalose didn’t work (it did, thankfully)

Miralax is safe to use every day. It works differently than Colace, (not a laxative or stimulant) using the body’s own water to soften stool. Start out with an eighth of a tsp. and increase a little until you see results. Takes a couple of days to kick in. If stool gets too soft, reduce the amount. It is totally odorless and tasteless. Works great.